1. Field of the Invention
The present invention relates to a DC convertor for supplying a load with direct voltage pulses with a variable frequency. The DC convertor has input terminals for connection to a DC supply network and comprises a filter circuit arranged between the input terminals and the main circuit of the convertor, said filter circuit comprising a capacitive parallel branch and a first series inductor between the input terminals and the capacitive parallel branch.
2. The Prior Art
FIG. 1 shows a known DC convertor for controlling the power to a DC motor M. It is fed from a direct voltage network with the voltage U.sub.N and the internal inductance L.sub.N. It is connected to the network by means of the input terminals P and N, which, if the convertor is stationary, may be ordinary connecting terminals and, if the convertor is arranged in a vehicle, may consist of a pantograph or trolley pole. The convertor comprises a thyristor T provided with turn-off equipment and a freewheeling diode D. By periodic ignition and extinction of the thyristor T, the motor is supplied with direct voltage pulses. By varying the relation between the lengths of the conducting and non-conducting intervals of the thyristor, the average value of the direct voltage which is supplied to the motor can be controlled.
The DC convertor requires for its function a stiff supply voltage. Its input is therefore provided with a parallel capacitor C. This capacitor is normally necessary, especially in those cases where the convertor is mounted in a vehicle and is supplied by way of a contact line which has internal inductance.
As a rule a convertor of this type works with a variable pulse frequency. At the frequency values, inductance values and capacitance values occurring in practice, there is a risk that some working frequency may correspond to the resonance frequency of the circuit constituted by L.sub.N and C.
The current I of the DC convertor consists of current pulses with a certain pulse frequency. This current can be divided into a DC component I.sup.D and an AC component I.sup.AC with the same frequency as the pulse frequency (and a number of harmonics). At the resonance frequency of the circuit L.sub.N - C, the AC component I.sub.N.sup.AC of the current drawn from the source will be very great (infinite if the circuit is free of losses), as well also as the alternating voltage component of the voltage across the capacitor C. This causes considerable drawbacks in the form of the danger of signal and tele-disturbances and deteriorated working conditions for the DC convertor.
In order to avoid these disadvantages, it is known to arrange a series inductor L in the DC convertor. Its inductance is then added to the network inductance L.sub.N, and this results in a reduction of the resonance frequency. By making the inductance of L great, the resonance frequency can be made low. If then a lower limit is set to the working frequency of the DC convertor so that the working frequency always exceeds the resonance frequency by a certain margin, the above-mentioned problems of resonance are avoided.
However, this known circuit has considerable drawbacks. In a DC convertor the pulse length cannot in general be lower than a certain minimum value. The average voltage supplied to the motor M is proportional to the product of the pulse length and the pulse frequency. Since the pulse frequency in this known circuit cannot be reduced below a minimum value determined by the resonance frequency of the filter, the load voltage cannot be reduced below a certain least value. This means, for example, heavy jerks when starting a vehicle driven by an electric motor. It has proven to be impossible for practical and economic reasons to increase the inductance of L (and thus decrease the resonance frequency) to such an extent that these drawbacks are avoided, since L must then have very large dimensions.